Reduce the size of the log produced by a migration failure or old database not yet migrated

for a single migration erreur, you may get from 5k lines to 22k lines of logs, repeating mostly the same things. All these lines are mostly issued by macros, plugins, and access to preferences. Each failure logs the whole exception again.
To sensibly reduce the logs, I propose that we lower the logging level of access to Preferences (user, space and wiki) to DEBUG. This will reduced the resulting log by more than 90%.

see http://markmail.org/thread/wa24cnqgqblxwkoa for discussion